A known type of low vacuum orthopaedic cement mixer is described in Puderbaugh et al, U.S. Pat. No. 4,185,072, which operates at very low vacuum levels ranging from 80 to 120 mm of mercury in order to remove the monomer vapor fumes and the odor associated therewith from the surgical suite during mixing. Prior to this, mixing was accomplished by hand spatulation using a paddle in an open vessel.
It is well known that in many orthopedic surgical. procedures it is necessary to employ a cement or grouting type agent, such as for attaching artificial joint implants, repairing or forming joints in bones, or other forms of orthopaedic work. The type of cement generally used for these purposes are self-curing resins formed from the blending of a wide variety of liquid monomers or comonomers with powdered polymers or copolymers to form a viscous admixture to be used as the grouting agent. When set, the resulting cements contain poly(methyacrylic acid esters) as their main ingredient. The powder, and subsequently the cement, can also contain such things as radiopacifiers, antibiotics, plasticizers, crosslinking agents, and compositing reinforcing fibers or beads.
The admixture of the powder and liquid components develops a quick setting material and preparation of the cement usually occurs directly within the operating theater just prior to use. This mixing must occur rapidly so the viscous admixture can be utilized in the orthopaedic procedure before autopolymerization occurs and the cement sets hard.
In the powder component, which can be an air-fluffed powder, air (upward of 50 percent by volume) is usually desirable around the powder particles to readily enable the liquid to wet the powder. We have recognized that if this necessary air is not subsequently removed from the viscous admixture, it will end up as small to medium sized pores within the set cement. Prior to the present invention removal of air or deaeration of the cement was not possible.
There are a number of sources of air bubbles within the cement admixture, such as from the air present in the powder component which can become entrapped upon blending with the liquid, or when the admixture becomes lumpy during mixing and the lumps themselves coalese thereby allowing bubbles to be formed there between. The dragging or moving of the paddle through the mixture also can create bubbles within the mix due to folding in of external air as was found to be the case with known paddle designs. Moreover, known paddle designs may cause the small and medium bubbles to coalesce into large bubbles. Also bubbles of monomer vapor may be present in the admixture.
The cement must be uniformly and thoroughly mixed in a relatively short amount of time so as to be as homogenous as possible and, preferably, without any entrapped gas in the form of air or monomer vapor. Mixing of the cement components may incorporate gas within the mixture. This entrapped gas will have to be removed in order to subsequently develop a set cement product that is not porous and exhibits desirably increased static and dynamic mechanical strengths.
We prefer to remove as much of the entrapped gas from the mixture as mixing proceeds as is possible so that few, if any, entrapped bubbles will remain within the viscous admixture thereby also eliminating porosity in the set cement. Were such bubbles to remain within the mixture, the mechanical properties of the set cement would not be as desired and a fracture might develop in the hardened cement at the points where such bubbles, entrapped air spaces or pores were located. Also, because some monomer vapor fumes might be generated which can be noxious and/or toxic in nature, and because mixing can often be carried out for a period of several minutes to assure a uniform mixture, it is desirable to filter the gas being removed.
The present invention is primarily concerned with a device that will more thoroughly mix the components under partial vacuum conditions of approximately 550 mm of mercury which provides the ability to deaerate or remove incorporated gas both from the spaces between powder particles prior to blending as well as from the mixing components during blending. The invention employs a mixing paddle designed to avoid dragging gas bubbles through the as mixing proceeds. Further, following initial blending, and with continued back and forth movement in clockwise and counterclockwise directions, the paddle interacts with the mixing components and creates flow and mixture movement conditions within the mixing vessel that aids the removal of any remaining gas bubbles. The mixing blades, when moved through the components, develop very large exposed surface areas within the mixture against which gas bubbles can be brought and there burst. The present invention also necessarily contemplates device that can withstand substantially higher vacuums than has heretofore been desirable.
The present invention is comprised of a modified reaction-mixing vessel, from that described in Puderbaugh et al, having an outer housing that is substantially reinforced in order to withstand high vacuums, a modified sealing cover structure, and a greatly modified mixing paddle. The apparatus described in Puderbaugh et al required use of an air vent or a loose fitting lid so that the interior of the reaction-mixing vessel would receive enough of an air flow so that excessive vacuum conditions within the apparatus would specifically be avoided. The present invention seeks just the opposite, that is, to provide a reaction-mixing vessel in which it is possible to develop a high partial vacuum and the avoidance of leaks of outside air therein.
Additionally, it has been found that the mixing blades, beaters or vanes used in prior art devices have not suitably mixed the materials nor have they handled or manipulated the components being mixed within the mixing chamber such that the mixture could be exposed, to the greatest extent possible, to the partial vacuum conditions within the mixing chamber. Further, prior mixing devices did not develop a circulation flow path that would move the mixing components upward and downward in the center of the mixing chamber, nor radially relative to the sidewall or along the sidewall of the mixing device, all of which assist in deaerating the viscous admixture.
After the liquid has been placed in the mixing vessel and the powder admixed into that liquid, wetting will have begun but some of the powder will continue to float on the liquid surface. By applying a partial vacuum after closing the vessel, some of the entrapped gas within the floating powder, but not all, will be removed.
For the entrapped gas bubbles incorporated within the mixed components to be removed, the pores or bubbles must be broken by the paddle structure or opened by vacuum conditions within the mixing vessel. In order for this to be accomplished the bubbles must be brought very close to or exposed on an exposed surface of the admixture. At the same time, it is not desirable to create larger pores or bubbles. Mixing blades that will cause or allow small gas bubbles to coalesce into larger bubbles will not aid in ridding the mixture of bubbles and might well further degrade the mechanical properties of the set cement product. It was found to be necessary, therefore, that large surface areas of the mixture be continuously developed and reformed during mixing so that entrapped or incorporated bubbles will be brought to an exposed surface of the mixture. When that occurs the entrapped gas bubbles can burst either by action of the mixing device or by exposure to the relatively high vacuum conditions within the vessel. It must also be understood that the vacuum level is preferably controlled to lie within a desired optimal range of about 500 to about 600 mm of mercury. If the applied vacuum is too low the porosity-causing gases cannot be effectively removed from the viscous admixture. Conversely, if the vacuum is too high, the liquid monomer will boil and create additional porosity problems.
The mixing paddle has a unique design and creates specific flow patterns within the mixture, depending upon the direction of rotation, which can occur in both clockwise and counterclockwise direction. The paddle provides not only much better mixing but a circulating flow which assures that, to the greatest extent possible, large and continuously changing surface areas of the mixed components become exposed to the vacuum conditions within the reaction-mixing vessel so that bubbles otherwise entrapped throughout the mixture will themselves be exposed and the gas released. Further, the paddle structure will not coalesce or merge the bubbles together as mixing progresses but rather allows the incorporated gas in the form of small bubbles to be treated equally as well as large bubbles.
The mixing device herein is designed to mix a variety of types of bone cements such as, for example, the mixing of self-curing resins used primarily for the internal orthopedic endoprostheses, as may be produced from the blending of a wide variety of monomers or comonomers with polymers or copolymers, which may produce polymethacrylic acid esters as the main ingredient in the set cement.
The mixer has also been designed to mix different quantities of bone cement, that is, single batches and double batches, not uncommonly 50 grams to 120 grams of admixture of said bone cement.
Other objects, features, and characteristics of the present invention, as well as the methods and operation and functions of the related elements of the structure, and to the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.